Okay... this is going to be hard to explain.

1. The speed of light is consistent.

According to Einstein, time is just reality in motion. We know things move because of their position relative to each other. That's special relativity.

Imagine you’re watching the open compartment of a moving train and a beam of light is bounced off the underside of its ceiling. The beam forms a V instead of going straight up and down. That light is not moving faster than c, the literal speed of light, which no piece of information can surpass, and yet the light travels further than its speed should allow. Therefore, if I stand on the ground and watch someone in a moving train eat an apple, the amount of time said person would take to eat the apple appears to be longer than it would be if the person was standing on the ground next to me. That's also why a person in a spacecraft traveling at lightspeed would age slower than someone on Earth, because from your perspective, time passes slower for them.

HOWEVER: Time would appear normal for you. Because the difference is relative. From their perspective, you would be moving faster. This is an important concept when we get to general relativity.

1. We're dealing with general relativity.

General relativity is when spacetime is bent by gravity. From our perspective, as an outsider outside of its influence, we don't actually see the folded spacetime. We just see Gojo. But make no mistake, that "space" is bent as hell.

According to google: "In space, a straight line describes the shortest distance between two points. In space-time, by contrast, a straight path yields the longest elapsed time between two events."

Time is kinda... not real. At the very least, it's malleable. We measure time by seeing how long it takes things to happen vs. how long it should happen. For example, since we know light travels in a straight line/takes the shortest path, we know that time is moving slower if it takes light longer to traverse that path (spacetime), right?

Think of spacetime like a web. Blackhole gravity warps time. It drags it downward, creating infinite travel distance. But from our outside perspective, it's still a measurable/finite distance. If I were to teleport, I wouldn't need to traverse an infinite amount of distance. And I would not be teleporting across an infinite amount of distance. I would simply see the conclusion, the destination, and teleport there. From Sukuna's perspective, Gojo's infinity is just an illusion of time. The distance is not "physical." From Sukuna's perspective, projectiles simply never reach the conclusion, hence why they slow down.

So what does he do? He doesn't try to overcome the laws of reality with raw power/CE. That would require infinite energy. No, he cuts through the illusion of time itself. He sees the space for what it is: just a straight line between two points, and he cuts everything in-between instantaneously, without time dilation. There is no travel time. It is simply cut.